Recently, power adapters become indispensable devices for many kinds of electronic devices. A power adapter usually has an AC-to-DC converter for rectifying and converting utility power into DC power required for powering a load such as a notebook computer or a mobile phone charger.
Referring to FIG. 1, a schematic circuit block diagram of a typical power supply system is illustrated. The power adapter 10 has a first terminal connected to utility power source 11 through an AC plug (not shown) and a second terminal connected to a load 12 through a connector (not shown). Within the power adapter 10, an AC-to-DC converter 101 is used for receiving the AC power from the utility power source 11 and rectifying and converting the utility power into DC power required for powering the load 12.
Although the AC-to-DC converter 101 of the power adapter 10 can rectify and convert the utility power into the DC power required for powering the load 12, there are still some drawbacks. For example, for carrying the load 12 (for example a portable electronic device such as a notebook computer) to other places, the user may simply separate the load 12 from the power adapter 10 while the power adapter 10 is still connected to the utility power source 11. Under this circumstance, the utility power is continuously transmitted from the utility power source 11 to the power adapter 10. Due to the line impedance between the utility power source 11 and the power adapter 10 or the continuous conversion of the AC power into DC power by the power adapter 10, the power adapter 10 is readily suffered from additional power loss. Nowadays, with increasing awareness of environmental protection, more and more products are designed in views of low power consumption, low power loss and high performance. As a result, the current power adapter is not satisfied.
Therefore, there is a need of providing a power conversion circuit for reducing power loss to obviate the drawbacks encountered from the prior art.